This application is based upon and claims benefit of priority of Japanese Patent Applications No. Hei-11-221325 filed on Aug. 4, 1999, No. Hei-11-250790 filed on Sep. 3, 1999, and No. 2000-178787 filed on Jun. 14, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a display device that has a self-luminescent display panel, such as an organic electroluminescent display panel, and more particularly to such a display panel having plural luminescent segments or sections.
2. Description of Related Art
A display device having an organic electroluminescent panel is generally known. En example of this kind of display device is disclosed in JP-A-10-222127. The display device disclosed therein has a dot-matrix-type organic electroluminescent panel. It is also known that luminance, or brightness of a picture element is substantially proportional to electric current supplied thereto. Therefore, the display panel is usually driven with constant current.
As opposed to the dot-matrix type organic electroluminescent display panel, a segment-type display panel usually includes plural luminescent segments, a luminescent area of which is different from each other. Accordingly, it is necessary to drive each segment with an amount of current proportional to its luminescent area to obtain uniform luminance among plural segments. In other words, it is necessary to supply an equal current density to each segment. If a number of current setters, each corresponding to each luminescent segment, are provided in a device for obtaining the uniform luminance, the device becomes complex and costly.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved display device in which a uniform luminance among plural luminescent segments or elements is realized without making the device complex and costly.
The display device is composed of a display panel having plural luminescent elements or segments, such as organic electroluminescent elements, and a driving circuit for supplying current to the elements. The driving circuit may be an integrated circuit (an IC circuit). The plural luminescent elements are grouped into several groups or sections, so that only the elements having the same or similar luminescent area belong to a given group. A circuit for setting an amount of current to be supplied to the elements is provided for each group. The amount of the current is set substantially in proportion to the luminescent area of the elements belonging to that group by selecting a resistance value of a variable resistor connected to the current setting circuit, or by setting a voltage of a voltage setter connected thereto. The current supply circuit may be composed of a push-pull circuit consisting of two transistors. Thus, the current density supplied to each luminescent elements is substantially all equal, and accordingly an equal and uniform luminance among all the elements is realized.
In case the luminescent elements, the luminescent area of which is similar to but somewhat different from one another, belong to a group or section, a current supply duty ratio for each element is adjusted according to the luminescent area of the element. In this manner, an equal luminance among the elements is attained though the same amount of current is supplied to all the elements in that group.
Alternatively, a resistor may be connected in parallel to each luminescent segment having the luminescent area different from one another. The resistance value of the parallel resistor is determined to equalize the current density supplied to all the segments. In this manner, an equal luminance among all the segments can be realized even when the same amount of current is supplied to the parallel circuit composed of the luminescent segment and the resistor. Auxiliary luminescent segments may be used in place of the parallel resistors. The size of the auxiliary segments is determined to equalize the current density supplied to the luminescent segments. In this case, the segments for displaying images are disposed in a display region of the panel while the auxiliary segments are disposed outside the display region.
According to the present invention, the current supply circuit in the display device is simplified by grouping the elements and thereby reducing the number of the current amount setters, while achieving an equal luminance among all the luminescent elements.